1. Field
Embodiments of the invention relate to radio frequency (RF) electronic systems, and in particular, to power amplifier systems.
2. Description of the Related Technology
Power amplifiers can be included in mobile devices to amplify a RF signal for transmission via an antenna. For example, in mobile devices having a time division multiple access (TDMA) architecture, such as those found in Global System for Mobile Communications (GSM), code division multiple access (CDMA), and wideband code division multiple access (W-CDMA) systems, a power amplifier can be used to amplify a RF signal having a relatively low power. It can be important to manage the amplification of a RF signal, as a desired transmit power level can depend on how far the user is away from a base station and/or the mobile environment. Power amplifiers can also be employed to aid in regulating the power level of the RF signal over time, so as to prevent signal interference from transmission during an assigned receive time slot.
When a power amplifier is configured to amplify a RF signal having a signal envelope that changes with time, such as a CDMA, W-CDMA or Long Term Evolution (LTE) signal, a usage profile of a mobile phone by a user can follow a probability density function in which it is more likely that the power amplifier will be operated below a maximum power level. The usage profile can vary with carrier system design as well as with operating conditions, such as a rural or urban operating environment. Since conventional power amplifiers suffer from reduced efficiency and linearity at low power levels, there is a need for improved power amplifier topologies. Furthermore, to prolong battery life of a mobile device, it can also be desirable to achieve a relative low quiescent current when the phone is in an idle condition.
To aid in improving the operation of the power amplifier system, including, for example, increased efficiency, improved linearity, and/or a reduced quiescent current, a power amplifier system can include multiple power amplifiers. However, a power amplifier system including multiple power amplifiers can be prone to instability for a variety of reasons, including, for example, the presence of feedback paths associated with the power amplifiers. Power amplifier instability can degrade system performance, and can lead to dropped calls and/or spurious emissions failures.
There is a need for improved power amplifier systems.